Impacts of Vegetation and Climate Change on Dryland Rivers: Lesssons from the Rio Puerco, New Mexico

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Summary: The arid American West relies on rivers for much of its water. Pressures from population growth, climate change, and invasive plant species threaten to transform the nation's dryland rivers, their water resources, and their ecosystems in ways that have been traditionally difficult to foresee. Sustainable, adaptive management of these resources requires an ability to predict how they will respond to perturbations. Because future drivers are likely to reach well beyond the conditions that have been observed in the past, it is critical to have process-based models that can make useful, transferrable predictions beyond the historical range of variability. The validity of such process-based models can only be demonstrated, however, by comparison to well constrained case studies of past and present behavior. We propose to create a unique new database that will provide a valuable opportunity to study the causes and consequences of river transformation over time scales of years to decades. One of the best studied and most dynamic basins in the southwest, the Rio Puerco, New Mexico, provides the springboard for our analysis. Over the past century, the Rio Puerco and its floodplains have undergone major changes in hydrology, sediment yield, and vegetation. Many of these changes are linked with the introduction of the exotic tree species tamarix in the 1920s, and its subsequent spread throughout the valley network. More recently, efforts to destroy tamarix have led to dramatic changes in channel geometry and sediment yield. Thus, the Rio Puerco represents a unique natural experiment in the effects of long-term vegetation change on a dynamic fluvial system. To take advantage of this experiment, we propose to build and analyze a database that integrates historical flow and sediment data, time series measurements of channel cross-section form, modern and historic vegetation patterns, flood sedimentation patterns, and high-resolution measurement of recent channel change derived from lidar imagery. Our analysis encompasses two timescales: (1) the event scale, addressing the impact of a large flood with pre- and post-event lidar, and (2) the decadal scale, focusing on the response to tamarisk spread from 1930s to present. In addition to providing new insight into river transformation, the integrated database will provide the scientific community with a unique platform for testing and refining the next generation of coupled flow-sediment-vegetation models.Intellectual Merit: Dryland river environments embody a rich set of feedbacks among flood flow, riparian vegetation, channel geometry, fine-sediment erosion, reservoir siltation, in-stream infiltration, and water quality. By exploring the system in the context of an extensively studied test case, this project will illuminate the nature and magnitude of these feedbacks. The project will also provide a first test of the hypothesis that it is possible to compute, from basic principles, the response of a dryland river system to perturbations in water input and vegetation cover on annual to multi-decadal time scales.Broader Impacts: The project closely integrates research, education, and outreach through a program of (1) partnership with K-12 teachers in bringing the exciting science of river dynamics into the classroom, (2) involvement of undergraduate and graduate students in interdisciplinary research, and (3) broader outreach through development, and distribution of study packages that include data analysis exercises in water, sediment, and vegetation trends over time, and model animations that illustrate processes and feedbacks. We also address an important societal need to predict potential consequences of environmental management actions, such as the recent introduction of tamarisk leaf beetle as a biocontrol agent. Finally, we will produce and distribute a detailed, quantitative database for benchmarking future models, on time scales ranging from a single flood event to eight decades of river transformation.